**3.4. Biomagnification behaviour of POPs in the Galapagos food-chain**

It is well recognized that the increase in organic chemical concentrations in lipids of organisms with increasing trophic level in food-webs originates from the magnification of the chemical concentration in the gastro-intestinal tract caused by food digestion and absorption [5,14]. In this study, the biomagnification capacity of organochlorine contaminants in the tropical food chain of the Galapagos sea lion is established (i.e. C*PREDATOR*>C*PREY*, BMF > 1).

However, a range of various factors directly or indirectly affect magnification process in predators, including animal ecologies and physiologies, feeding preferences, life history parameters (sex, age, body size and corporal condition), reproduction, geographic locations and stochastic-climatic events. Furthermore, the composition of contaminants can be shaped through toxicokinetics processes (i.e., uptake, metabolism, respiration and excretion), influencing the persistence and food-web biomagnification of POPs. Due to these factors, it is complex to elucidate whether a wild predator is at a steady state with its diet; therefore, calculated BMFs may not always reflect actual biomagnification [54]. As shown in this study, predator-prey BMFs revealed the biomagnification capacity of POPs in the food chain of the Galapagos sea lions, which is an apex predator possessing flexible feeding preferences (dietary plasticity).

Efficient uptake and dietary assimilation and slow depuration/excretion rates of these compounds (PCBs with KOW ranging 105−107, and OC pesticides KOW ranging 103.8−107.0) explain the high degree of biomagnification in the Galapagos marine food chain. Dietary absorption efficiencies of Penta and Hexachlorobiphenyls are typically between 50-80% in fish and 90-100% in mammals [55] and chemical half-lives (*t*1/2) for recalcitrant PCBs such as PCB 153 in organisms exceed 1000 days [56]. The analysis of BMFTL estimates of PCBs and OC pesticides (Figures 4-5) indicates that OC pesticides and PCBs are accumulated by fish and sea lions and also biomagnify in the food chain. Based on contaminants' predator-prey BMFs, the DDT metabolites, *p*,*p*'-DDT and *p*,*p*'-DDE, followed by *trans*-nonachlor (Figure 4), are the most bioaccumulative pesticides, while PCB 74 and 153 are the most bioaccumulative PCB congeners in the Galapagos sea lion (Figures 5). The less bioaccumulative compounds are *trans*-chlordane and PCB 156.

Of particular importance is the biomagnification behaviour of *β*-HCH with a KOW< 104 (KOW = 103.8; Figure 4b,d), but with a KOA of 108.9−1010.5 (Figure 4a,c), contrasting with the regulatory criteria and current management policies (i.e. Stockholm Convention; CEPA) for POPs that consider only chemicals with KOW values >105 as bioaccumulative substances [7]. The predatorprey biomagnification factors (BMFTL = 63−552) of *β*-HCH in Galapagos sea lions exceed equivalent biomagnification factors of PCB 153 (BMFTL =18.0−72.2) and PCB 74 (BMFTL =30.0−72.0), as shown in Table 2. This portrays that *β*-HCH, a relatively hydrophilic and nonmetabolizable chemical, biomagnifies in the tropical marine mammalian food chain of an air breathing organism (the Galapagos sea lions), which is explained by the relatively high KOAof *β*-HCH (KOA> 107.0) and its negligible respiratory elimination. Biomagnification of *β*-HCH was evident in the lichen-caribou-wolf terrestrial food chain, in the maritime and interior grizzly bears' food chains, and in a marine mammalian food web (including water-respiring and airbreathing organisms) from temperate regions of Canada and the Canadian Arctic [2,14,19].
